1. Field of the Invention
The present invention relates to a calibration method for a multi-component force measuring spindle unit used in a tire testing machine to calibrate the spindle unit which simultaneously measures, in a region of a support shaft of a tire to be measured, loads in a plurality of directions derived from the tire.
2. Description of the Related Art
Tire testing machines have been conventionally used to measure dynamic characteristics, such as, for example, a rolling resistance, of a tire in a running condition. Such a tire testing machine is equipped with a multi-component force measuring spindle unit which is adapted to rotatably support, on a spindle shaft, the tire to be measured, and capable of simultaneously measuring loads derived from the tire in a plurality of directions on a region of the spindle shaft.
The tire attached to the multi-component force measuring spindle unit is placed under a predetermined load on an outer circumference of a running drum installed in the tire testing machine, to simultaneously measure loads exerted in each direction on a spindle shaft under various conditions of a camber or slip angle of the tire, a grounding load, and other factors by means of “a multi-component force measuring sensor (a load cell)” installed in the multi-component force measuring spindle unit.
Then, an actual load acting on the tire is calculated from the measured load.
It should be noted that the term “load” as used herein shall include a moment. For example, when a direction along which a tire is pushed against the running drum is defined as a Z axis, a tire traveling direction is defined as an x axis, and a direction of an axis of a tire rotating shaft (a spindle shaft) is defined as a y axis, actual loads acting on the tire may include a tire grounding load Fz, a tire rolling resistance (tractive force) Fx, a tire lateral force (cornering force) Fy, a self-aligning torque Mz which is a turning moment about the z axis, an overturning moment Mx which is a turning moment about the x axis, and a rolling resistance moment My which is a turning moment about the y axis.
The “multi-component force measuring sensor” installed in the above-described multi-component force measuring spindle unit may have various structures, including those described in Japanese Patent Laid-Open Publication Nos. S57-169643-A (Patent Document 1) and S52-133270-A (Patent Document 2), and U.S. Pat. No. 4,821,582 (Patent Document 3).
For example, when a spindle unit employing a load cell as disclosed in Patent Document 1 is used, it becomes possible to measure six components of force resulting from a tire on the spindle shaft. However, in the spindle unit disclosed in Patent Document 1, because a tire load is exerted on a position deviated from a main body of a multi-component force measuring sensor, it is feared that a moment exerted on the multi-component force measuring sensor is increased, resulting in a situation where a sufficient load-carrying capacity is not obtained.
On the other hand, multi-component force measuring sensors described in Patent Document 2 and Patent Document 3 are able to overcome the problem associated with the multi-component force measuring sensor of Patent Document 1.
More specifically, in spindle units of Patent Documents 2 and 3, two multi-component force measuring sensors are disposed at a predetermined distance from each other along an axis direction of a spindle shaft. In particular, according to a structure of the spindle unit in Patent Document 3 in which the two multi-component force measuring sensors are coupled through a cylindrical member (a sleeve) having high stiffness, a moment or other forces exerted on the multi-component force measuring sensors can be reduced, and a sufficient load-carrying capacity can be accordingly obtained.
In the thus-constructed spindle unit, however, both translation and rotation are constrained between the two multi-component force measuring sensors. In other words, the spindle unit is in a statically indeterminate state or an excessively constrained state. As a result, when a tire test is conducted, measured loads obtained by the two multi-component force measuring sensors can not be determined simply from a balancing condition of an external force, and will be influenced by a condition of a deflection or a deflection angle which is determined based on a stiffness relationship between the cylindrical member and the multi-component force measuring sensors.
Although it can be considered that the multi-component force measuring sensors are integrally constructed with the cylindrical member to reduce an effect (adverse effect) as described above, the integral construction presents problems such as an increase in manufacturing cost or an increase in complexity of maintenance procedures. As long as they are structured as separate units, ease of maintenance, such as a capability of continuous use achieved only by replacing a defective part can be ensured, which is an extremely advantageous feature.
Further, because the two multi-component force measuring sensors and the cylindrical member have high stiffness in the excessively constrained state, the structure of the spindle unit as described in Patent Documents 2 and 3 has a problem of susceptibility to heat.
In fact, the spindle unit as described above suffers from a great internal force generated inside the unit by occurrence of even a slightest amount of thermal strain. More specifically, bearings that support the spindle shaft become a source of heat, and the heat is transferred to the entire unit, creating a certain distribution of temperature. As a result, the spindle unit will be deformed, and such thermal deformation of the spindle unit has an effect of forcing deformation on the multi-component force measuring sensors and causing the multi-component force measuring sensors to output a load which is not associated with the tire load. Thus, the effect emerges as an error in measurement.
In the structure of the spindle unit, as described above, including the two multi-component force measuring sensors which are disposed on positions spaced-apart from each other along the axis direction of the spindle shaft and respectively attached to the cylindrical member or the like, the influence of excessive constraint or thermal deformation may create a situation in which accurate measurement of the loads (such as a translational load and a moment) acting on the tire is difficult.
As a means for circumventing the situation, it is extremely effective to calibrate the multi-component force measuring spindle unit (to conduct a calibration test) prior to a tire test.